Explicitly return None for negative event indices. Prior to this,

eventat(-1) would return the next-to-last event causing the back button
to cycle back to the end of an event source instead of stopping at the
start.

tools/sched/schedgraph.py

@@ -856,7 +856,7 @@ def ysize(self):
 		return (Y_EVENTSOURCE)
 
 	def eventat(self, i):
-		if (i >= len(self.events)):
+		if (i >= len(self.events) or i < 0):
 			return (None)
 		event = self.events[i]
 		return (event)